This invention relates to an electrophotographic photosensitive member which possesses photoconductivity upon illumination with electromagnetic light in the infrared, visible, ultraviolet, X-ray, and .gamma.-ray region, and which permits an image to be formed after the formation of an electrostatic latent image.
In image forming technique, such as an electrophotography, or image pickup, use is made of a photoconductive material which shows photoconductivity upon illumination by light. Recently, attention has been paid to an amorphous silicon (hereinafter referred to as an a--Si) as a photoconductive material. In comparison with a photoconductive material selected from an inorganic material, such as Se, CdS, Se--Te alloy or Se--As alloy, or an organic material, such as a PVCz or TNF, the a--Si film has the advantages of having an excellent spectral sensitivity over the visible light range, a high surface hardness, and of being easy to handle, durable at a high temperature, and pollution-free. Furthermore, the a--Si film, if a high-frequency glow discharge decomposition method is used, can be formed with a larger area and a uniform thickness, without any film formation restrictions resulting from the shape and material of the substrate.
If the a--Si is used for the electrophotographic photosensitive member, since the resistivity, in the dark, of the a--Si (hereinafter referred to as dark resistivity) is usually of the order of 10.sup.8 to 10.sup.10 .OMEGA..multidot.cm, it is not possible in that instance to retain charges on the surface of the electrophotographic photosensitive member made from the a--Si. Therefore, attempts have been made to enhance the dark resistivity through the doping of a small quantity of an impurity element of Group III of the Periodic Table, such a B, Al, Ga and In, into a photoconductive layer (where photocarriers are generated) to thus enhance the charge retention capability of the photosensitive member. When using this technique, however, the charge retention capability of the photosensitive member is not sufficient, since it is difficult for the photoconductive layer alone to retain the charges at the charging time. It is, therefore, not possible to suppress the dark decay.
It may be possible, however, to sandwich the photoconductive layer with high-resistance insulating layers. When the photoconductive layer is electrified to form charges at the surface, they are retained by the high-resistance insulating layer at the surface of the photoconductive layer, and the transfer of the charges from the conductive substrate into the photoconductive layer is suppressed by the high-resistance insulating layer which is formed between the photoconductive layer and the conductive substrate. In this technique, however, a breakdown occurs due to the concentration of an electric field toward the high-resistance insulating layer, causing carriers to be stored at the interface between the high-resistance insulating layer and the photoconductive layer, with the result that residual potential is enhanced.